Verification of satellite signals of global navigation systems

30m High Gain Antenna of DLR at Weilheim

The Institute of Communications and Navigation operates in co-operation with the German Space Operation Center (GSOC) a verification facility for satellite signals. With the use of the facility signal of navigation systems (e.g. Galileo, GPS, GLONASS, COMPASS) can be highly precise received and analyzed.

Because the slightly low signal level of the navigation satellites and the great distance between the satellite and the user on ground (more than 20000 km) an antenna is needed with a small aperture angle and a high antenna gain for the precise investigation of the radiated signals. DLR can use a 30m dish antenna located at the ground station in Weilheim which is rarely facility in Germany and in Europe.

Based on the analyzed measurement raw data it is possible to compare the transmitted signals with the published specifications. In addition signal anomalies can be detected; the causal effects and the influence on the future navigation services can be worked out. Furthermore the institute is able to develop countermeasure regarding these effects.

The 30m High Gain Antenna:

In early 1970’s a 30 m high gain antenna was built in the near of Weilheim. The antenna was used for the space mission HELIOS A/B. HELIOS started in 1974 and was the first common interplanetary mission between German and USA. In the following years the antenna was used for different space missions like Giotto, AMPTE or Equator-S. The antenna is a Cassegrain type and has a gain of around 50dB and an antenna beam width below 0.5° in the L band which is used by the navigation satellites.

The measurement equipment:

The measurement system consists of a new developed feed for circularly polarized signals in the L band. The received signals are firstly amplified by a low-noise amplifier and than runs through a complex measuring system. The measurement setup provides the ability to record the signal using different filters for suppressing interferences out of the interested band widths. An online calibration is built in, which is essential for accurate signal analysis. The signals are recorded by a vector signal analyzer and stored in a data unit providing the raw date for the signal processing and analysis.

High sensitive measurement equipment with broad band feed and signal analysator

Actual signal analysis:

Employees of DLR investigate not only the satellite signals of the future European navigation system Galileo (GIOVE A/B, IOV and FOC) but also other existing or new global satellite systems like GPS, GLONASS and COMPASS.

GLONASS:

On 26th February the Russian Space Forces launched the first satellite of the new GLONASS-K generation. With this new satellite the Russian system is able to provide for the first time a CDMA signal in addition to the traditional FDMA signals to the users. This upcoming CDMA signal is located at the L3 frequency band with the center frequency at 1202.025 MHz and will be used for test purposes only. As future steps Russia plans to implement further CDMA signals at L1, L2 and later L5 frequencies at their navigation satellites to achieve compatibility and interoperability to GPS and Galileo.

Measured spectrum and time signal of GLONASS L3 signal

GPS:

End of March 2009 a new type of GPS satellite started: Satellite IIR-20M. This satellite transmitted the new GPS L5 signal for the first time, which was intensively examined by the institute.

Measured spectrum and time signal of GPS L5 signal

Galileo:

GIOVE-B

In spring 2008, DLR supported the European Space Agency by the “In Orbit test” of the second Galileo test satellite GIOVE-B with high precision calibrated measurements. Main issue was focused not only on the detection of the correct function of the satellite after launch but also on the analysis of its long-term stability.

The new GIOVE-B L1 CBOC signal (spectrum, IQ diagram, time signal)

GIOVE-A

At the beginning of 2006 the Institute of Communications and Navigation received the signals of the new European navigation satellite GIOVE-A as one of the first institutions worldwide. The signal were recorded with highest accuracy and analyzed by the group.

GIOVE-A L1 signal shortly after the launch of the satellite

With the help of the obtained measurement and analysis data imperfections of the transmitted signal of GIOVE-A were be worked out. These effects were probably based on dispersion and non linearity’s and countermeasures were proposed. These results have helped to adjust the signal transmitter by the ESA and to optimize the future European navigation system Galileo.